1. Field of the Invention
The present invention, in general, is related to steam turbines, gas expanders, and industrial gas turbines and, more particularly, to a seal for sealing the radial gap between the rotor and the casing of the turbine.
2. Description of Related Art
With reference to FIGS. 1 and 2, a typical steam turbine 1 includes a casing 3 having an inlet end 5 and an exhaust end 7 and a rotor 9 extending through inlet end 5 and exhaust end 7 of casing 3. Packing boxes 11 are provided at inlet end 5 and exhaust end 7 of casing 3 around rotor 9. Packing boxes 11 are employed to seal an annular gap formed by rotor 9 penetrating through inlet end 5 and exhaust end 7 of casing 3.
Traditionally, carbon ring seals 13 have been installed in packing boxes 11 to limit the leakage of gases from casing 3 to the atmosphere. A plurality of carbon rings 13 are positioned within grooves 15 in each of packing boxes 11. Typically, multiple carbon ring seals 13 are positioned within each of packing boxes 11.
Carbon is used in the manufacture of the ring seals 13 because it provides a low friction surface to interact with the rotor 9 and packing boxes 11. However, carbon is a very brittle material and may fracture if too great of a load is applied thereto. Therefore, industry standards require a limitation of the differential pressure across a carbon ring to 35 psi. As a result, with carbon ring seals it is typically necessary to provide multiple seals along the shaft.
Accordingly, a need exists for a ring seal for use in a packing box that ensures low leakage through the rotor shaft-casing gap while also accommodating radial motion of the seal for shaft excursions. A further need exists for a ring seal that is manufactured from a durable material to prevent fracture when encountering a large differential pressure load. Provided according to this invention is a brush seal that can be used with existing packing boxes to replace most or all of the carbon ring seals.